1. Field of the Invention
The present invention relates to a full-color imaging toner for use in a full-color image forming apparatus, such as full-color copying machine and full-color printer.
2. Description of the Related Art
Various kinds of image forming apparatuses, such as copying machine, printer, and facsimile, have been widely used such that an electrostatic latent image formed on a photosensitive member is developed with a toner into a toner image which in turn is transferred on a recording medium, such as recording paper, for image formation. Recently, full-color image forming apparatuses, such as full-color copying machine and full-color printer, have been increasingly used in reproducing a full color image using a plurality of color toners.
Such full-color image forming apparatuses of various systems have been known in the art. For example, in one known system, an electrostatic latent image corresponding to a predetermined color is formed on a charged photosensitive member by a writing means or the like which irradiates a light beam, such as laser beam, and a predetermined toner selected from magenta, cyan, and yellow toners and, when required, black toner, is supplied to the electrostatic latent image for development thereof. The resulting toner image transferred onto a recording medium, such as recording paper, held on an intermediate transfer member. The steps of forming such a latent image, developing the latent image into a toner image, and transferring the toner image are carried out for each respective color to form a full-color toner image such that toner images of respective colors are placed in superposed relation on the recording paper, the toner image being then fixed on there recording paper. In another known system, a predetermined toner selected from magenta, cyan, and yellow toners and, when required, black toner, is supplied to an electrostatic latent image corresponding to a predetermined color which is formed on the photosensitive member for development as above described, and the resulting toner image is transferred onto an intermediate transfer member. The steps of forming such a latent image, developing the latent image into a toner image, and transferring the toner image are carried out for each respective color to form a full-color toner image such that toner images of respective colors are placed in superposed relation on the intermediate transfer member. Thereafter, the full-color toner image is transferred from the intermediate transfer member to a recording medium so that the toner image is fixed on the recording paper.
For transfer of a toner image formed on the photosensitive member or the like to a recording medium, there has been known a transfer device such that, as described in Japanese Patent Application Laid-Open No. Sho59-46664, a bias voltage-applied roller is brought into pressure engagement with a photosensitive member to form a nip between the transfer roller and the photosensitive member through which a recording sheet is caused to pass so that the toner image on the photosensitive member is press-transferred onto the recording sheet. Use of such a transfer device makes it possible to reduce out-of-sync possibility of recording-paper transport and transfer deviation and, in addition, to reduce the length of the transport path for recording paper and the size of the photosensitive member to thereby achieve the size reduction of the image forming apparatus.
However, the use of the transfer device poses a problem that since the transfer roller is held in pressure engagement with the photosensitive member on which a toner image has been formed, the toner is forcibly pressed against the photosensitive member into adhesion thereto, so that the toner at a center portion of the toner image is not satisfactorily transferred onto the recording paper. This may often result in an image defect such that a so-called image break trouble occurs with respect to the image formed.
In case that such a transfer device is used in such a full-color image forming apparatus as described above, an image break due to such erroneous transfer as aforesaid tends to occur because of the fact that, as above stated, magenta, cyan, yellow toners and, when required, black toner are respectively transferred so that toner images of respective colors are placed in superposed relation. More especially where, after toner images of respective colors are press-transferred onto the intermediate transfer member, the transferred toner image on the intermediate transfer member is further press-transferred onto recording paper, an image break trouble is prone to occur in the course of toner image transfer from the intermediate transfer member to the recording paper, so that the image obtained has quite a number of image breaks.
Generally, in full-color image forming methods, wherein toner requirements are so high that toner supply must be replenished so often, quick rise is required to a predetermined charge level of each additional toner supply.
In the event that the toner charge should fall short of the requirement, or that the toner should be so excessively charged as to cause variations in toner charge, any smooth and uniform transfer of the toner image to the intermediate transfer member or the like could not be achieved, with the result that there would occur fogging and toner dusting which would, in turn, lead to image quality degradation.
Where the toner is triboelectrically charged in relation to a carrier or a charging member (a blade or sleeve), it is desirable that after the toner has been charged to a specified charge level, the charge should be kept constant. If the charge continues to increase after a charge rise to the predetermined charge level, the charge becomes larger than required. The presence of such a toner may cause toner particles to electrostatically adhere to the carrier and charging member to form an accumulation of toner particles. As a result, the chargeability of the carrier and charging member relative to the toner is lowered to cause variations in toner charge, and this will inevitably lead to the occurrence of aforesaid troubles, such as fogging and dusting. Therefore, it is required that once the toner is charged to a specified level, the toner charge must be kept stable at the specified level.
The above mentioned problem is more pronounced in a full-color image forming method for solid image formation in particular wherein the toner is used in an increased concentration relative to the carrier.
Further, in view of the fact that the toner charge is subject to variations depending upon the environmental conditions in which the toner is placed, it is required that the toner is environmentally stable.